Fluorescent lamps have "negative resistance." This means that the operating voltage decreases as power dissipation in the lamp increases. Therefore, a circuit for supplying power to a fluorescent lamp requires a controllable alternating current power supply and a feedback loop that accurately monitors a current signal in the lamp in order to maintain operating stability of the circuit and to have an ability to control the lamp brightness. Such a circuit for supplying power to a fluorescent lamp may comprise a transformer wherein the lamp is coupled to a secondary winding of the transformer and is isolated from the rest of the circuit, including the sensing circuit, by the transformer. This makes directly sensing the lamp current signal difficult. Therefore, what is needed is a circuit for accurately sensing the current in a fluorescent lamp that is isolated from the sensing circuit by a transformer.
One such known circuit utilizes a resistor coupled in series with the transformer primary winding. During operation, a voltage signal across the resistor is monitored for utilization by the feedback loop. This voltage signal is multiplied by the resistance value to determine the current signal in the primary winding of the transformer. The current signal in the secondary winding is assumed to relate to the current signal in the primary winding by the ratio of turns between the primary and secondary windings. Therefore, the current signal in the secondary winding of the transformer is sensed indirectly by sensing the voltage signal across the resistor coupled in series with the primary winding.
Transformers, however, suffer from an operational characteristic that the above-described current sensing technique does not take into account. A Magnetization current of a transformer is the current required to produce magnetic flux in the transformer core. FIG. 1 shows a schematic diagram of an approximate equivalent transformer circuit which takes into account the magnetization current. In FIG. 1, the transformer comprises a primary winding Lp and a secondary winding Ls. An inductor Lm coupled in parallel with the primary winding Lp models the effects of the magnetization current Im. It can be seen from FIG. 1 that when an ac voltage signal Vp is applied to the primary winding Lp, the resulting current signal Ip is divided into the magnetization current signal Im and the effective current signal Ie. The magnetization current signal Im lags the voltage signal Vp by 90 degrees. In addition, the magnetization current signal Ie does not directly contribute to inducing current to flow in the secondary winding Ls. Therefore, the above-described technique of sensing current in a secondary winding of a transformer suffers from error caused by not taking the magnetization current into account.
Therefore, what is needed is a circuit for sensing a current signal in a secondary winding of a transformer by monitoring a current signal in a primary winding of the transformer which accounts for the magnetization current of the transformer.